Device with aeration mitigation for improved measurement of fluids

ABSTRACT

A housing assembly for a fluid sensor assembly includes a housing having a first and second sensing volumes. A fluid port attached to the housing has a porous membrane covering first and second fluid apertures. The first sensing volume included a vertically-oriented waveguide and is fluidly coupled to an exterior of the housing through the first fluid aperture, where the first aperture port has an area smaller than an area of a cross section of the waveguide. The second sensing volume is coupled to the exterior of the housing through the second fluid aperture, the second fluid aperture having an area larger than a cross section of the second sensing volume. While the first sensing volume and the second sensing volume are both in fluid communication with an exterior of the housing, the fluid port and porous membrane internally isolate the first sensing volume from the second sensing volume.

BACKGROUND

Diesel Exhaust Fluid (DEF) tanks have become standard on diesel poweredground vehicles since 2010 in the United States of America. DEF is areduction agent that is an Aqueous Urea Solution (AUS) used in SelectiveCatalytic Reduction (SCR) diesel emissions systems. DEF has uniqueproperties and it critical to the performance of the emissions systemson diesel engine equipment equipped with SCR. As such, it is typical toprovide multiple sensors in the DEF reservoir. These sensors monitor thefluid level, fluid temperature and fluid concentration (urea in water).Multiple sensor strategies are used to discern the reservoir and fluidconditions. Most, if not all, sensor technologies (e.g. ultrasonic,thermal dispersion, infrared spectrographic etc.) are sensitive to airin the DEF. Entrained air (bubbles) changes the physical characteristicsof DEF as well as the behavior of the sensor. Regardless of theparameter being measured (speed of sound, resistance/conductance,specific heat, dynamic viscosity, optical characteristic etc.), air inthe fluid will impact the measurement.

The air bubbles that impact sensor function can be categorized into twotypes. First, volumes of trapped air in a confined space create largerbubbles (approximately 0.002 ml or greater in size). These largerbubbles have higher buoyancy and typically rise easily in the liquid.The second type of air bubbles are extremely small and referred to as“micro-bubbles” or “nano bubbles”. These “micro-bubbles” have relativelylow buoyancy and tend to attach to vertical and horizontal surfaces. Thebuoyancy forces of these micro-bubbles are inadequate to overcome thesurface adhesion. Accordingly, these “micro-bubbles” may adhere tosensor surfaces and sensor reflectors. Large bubbles are typicallyformed from trapped air in enclosed spaces that get filled with the DEF.This often occurs in sensors that utilize covers, shrouds or otherenclosures to provide a stable liquid environment for the sensor tooperate (i.e. minimize liquid slosh or movement). To prevent the trappedair, these enclosures may provide venting apertures at the highestpoints to allow the air to escape. Additionally, fill apertures areprovided at the lowest points to allow the liquid to enter the enclosureand displace the air. However, when these apertures are large enough toallow air to easily escape as liquid enters the shroud/enclosure, theyalso provide a path for “micro-bubbles” to be driven into theenclosures. Once inside the enclosure the “micro-bubbles” stick tocritical surfaces such as the reflector or sensor faces.

The smaller, “micro-bubbles”, are generated under extreme agitation oraeration of the DEF. Typical causes are aeration during DEF tank fillingand sloshing of DEF fluid due to the dynamic environment of a vehicle.Another source is the standard filling nozzle for DEF, which utilizes aventuri system to provide automatic shut-off of the nozzle when the DEFtank is full. This venturi system actually ingests air from the tank andentrains the air in the DEF as it goes through the nozzle into the DEFtank. For the most part, the entrained air eventually evolves out of theDEF and does not adversely impact on the system operation. However, someof the smaller (“micro-bubbles”) affix themselves to the internal wallsof the sensor enclosures and disrupt the sensor readings. Liquidagitation has been found to be inadequate to remove these bubbles. Themost reliable way to remove “micro-bubbles” is to manually wipe theimpacted surfaces and re-submerge the sensor. This is of courseimpractical for a sensor on an operating vehicle.

SUMMARY

In one example, a housing assembly for a fluid sensor assembly includesa housing having a first sensing volume and a second sensing volume. Afluid port is attached to the housing and has at least a first fluidaperture and second fluid aperture. At least one porous membrane isassembled between the fluid port and the housing, the at least oneporous membrane covering the first and second fluid apertures. The firstsensing volume comprises a vertically-oriented waveguide and is fluidlycoupled to an exterior of the housing through the first fluid aperture,where the first aperture port has an area smaller than an area of across section of the waveguide. The second sensing volume is coupled tothe exterior of the housing through the second fluid aperture, thesecond fluid aperture having an area larger than a cross section of thesecond sensing volume. While the first sensing volume and the secondsensing volume are both in fluid communication with an exterior of thehousing, the fluid port and porous membrane internally isolate the firstsensing volume from the second sensing volume.

The fluid port may be planar. The at least one porous membrane maycomprise a single sheet of porous membrane covering the first and secondfluid apertures. A gasket may be disposed on the porous membrane forsealing purposes. The gasket may internally isolate the first sensingvolume from the second sensing volume. These features may be combined.

The housing assembly may further comprising a first ultrasonic sensorunder the waveguide, a second ultrasonic sensor at a bottom of thesecond sensing volume, and a target located a fixed distance above thesecond ultrasonic sensor.

The first sensing volume may further comprise a first vent to theexterior of the housing assembly at a top of the waveguide, and thesecond sensing volume may further comprise a second vent located at atop of the second sensing volume into the first sensing volume.

In another example, a housing assembly for a fluid sensor assemblyincludes a housing having a first sensing volume and a second sensingvolume. The first sensing volume comprises a vertically orientedwaveguide having a first sensor at a bottom of the waveguide and a firstair vent to an exterior of the housing assembly at a top of thewaveguide. The first sensor volume is fluidly coupled to an exterior ofthe housing through a first fluid aperture covered by a first area ofporous membrane. The second sensing volume has a target at a fixeddistance from a second sensor and a second air vent at a top of thesecond sensing volume. The second air vent vents into the first sensorvolume. The second sensing volume is coupled to the exterior of thehousing through a second fluid aperture covered by a second area ofporous membrane. The first and second apertures do not provide internalfluid communication between the first sensing volume and the secondsensing volume.

The target may be located vertically above the second sensor. The secondsensing volume may further comprise an air accumulation volume above thetarget. The second air vent may be horizontal and provide a passage intothe waveguide from the air accumulation volume.

The first area of porous membrane and the second area of porous membranemay be provided on a single sheet of porous membrane covering the firstand second fluid apertures. The first and second fluid apertures may beprovided on a planar fluid port. The sheet of porous membrane may beattached to the housing assembly by the fluid port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first example of a DBF sensor assemblyaccording to an aspect of the invention.

FIG. 2 is a side view of the DEF sensor assembly of FIG. 1.

FIG. 3 is a side view of the DEF sensor assembly of FIG. 1 with a fluidport removed.

FIG. 4 is a perspective view of a second example of a DEF sensorassembly according to another aspect of the invention.

FIG. 5 is a side view of the DEF sensor assembly of FIG. 4.

FIG. 6 is a side view of the DEF sensor assembly of FIG. 4 with a fluidport removed.

FIG. 7 is a side view of a third example of a DEF sensor assemblyaccording to another aspect of the invention.

FIG. 8 is a side view of the DEF sensor assembly of FIG. 7 with a fluidport removed.

FIG. 9 is a cross section of the side view of the DEF sensor assembly ofFIG. 7.

DETAILED DESCRIPTION

A first example of a DEF sensor assembly 10 is illustrated in FIG. 1. ADEF sensor assembly 10 is typically installed in a DEF reservoir or tank(not illustrated). The DEF sensor assembly 10 may include a controllerhousing 12, a controller 14 and a wiring conduit 16. The controller 14is typically potted in waterproof material which has been omitted fromthe drawings to make the controller 14 visible. An adapter 18 isprovided on top of the wiring conduit. The adapter 18 may be threaded tofacilitate mounting in the DEF header or reservoir. Various provisionsmay be made for sensing DEF characteristics. For example, a fluid levelsensing waveguide 20, such as a tube as illustrated, may be provided.Providing such a waveguide improves accuracy because the fluid insidethe fluid level sensing waveguide 20 varies less than the DEF fluid tankin general, which may experience sloshing as a vehicle is in motion. Asensor element (not illustrated) is mounted under the level sensingwaveguide 20 and electrically coupled to the controller.

Referring to FIGS. 2 and 3, DEF concentration sensing housing 22 is alsoprovided. The concentration sensing housing 22 comprises a DEFconcentration sensor 24, a sensor reflector 26, and a housing fluid port28. Sensor reflector 26 is dimensioned to allow air to pass between itand at least one wall of concentration sensing housing 22. Below thesensor reflector 26 is a sensor volume 38 and above the reflector is anair accumulation volume 36.

“Micro-bubbles” disadvantageously stick to various surfaces and remainin the sensing volume regardless of venting provisions for largerbubbles. According to one aspect of the present invention,“micro-bubble” attachment is mitigated by preventing micro bubbles fromentering the sensing chamber in the first place. The housing fluid port28 includes an aperture 30 and a porous media 32. The porous media 32allows DEF fluid to fill the concentration sensing housing 22 throughthe aperture 30 while substantially blocking micro-bubbles from enteringthe chamber. In order to be effective, the porous media 32 should be theonly path for fluid to enter the chamber from the main reservoir volume,no other liquid access points should exist. Porous media 32 may bemounted on the housing fluid port 28. Housing fluid port 28 may includean O-ring style gasket or seal to prevent ingress of bubbles around thehousing fluid port 28. To further assist in preventing air from enteringthe chamber via the porous media 32, a hydrophilic media or treatmentapplied to the porous media 32 may be utilized. The hydrophilic aspectof the media encourages the media to be fully “wetted” to encourage freeflow of the liquid and to ensure that no air is trapped in the media. Asensor 24 is located underneath the concentration sensing housing 22 andelectronically coupled to the controller.

A fluid inlet 40 couples an interior volume of the concentration sensinghousing 22 to the level sensing waveguide 20 to allow filtered DEF fluidto fill the level sensing waveguide 20 from the concentration sensinghousing 22. The fluid inlet is located at the bottom of theconcentration sensing housing 22.

In one advantageous aspect of the present invention, an air accumulationvolume 36 is provided at the top of the concentration sensing housing 22above the sensor reflector 26 for the air to collect prior to exitingthe chamber. The air accumulation volume 36 provides a space for air tocollect away from the sensor components. This way any remaining air thatdoes not exit via the vent 42 will not interfere with the sensorreadings. The shape of the air accumulation volume 36 may vary, as longas it provides a guidance path to the air vent.

While porous media 32 prevents bubbles from entering concentrationsensing housing 22, it also prevents air bubbles from exiting thechamber. Accordingly, an air vent 42 may be provided at the top of theconcentration sensing housing 22 and into the level sensing waveguide 20to allow air to escape the accumulation volume 36. The top of theconcentration sensing housing 22 may be angled or sloped to improveevacuation of trapped air and air bubbles. A waveguide vent 34 isprovided at the top of the level sensing waveguide 20 to allow air toescape from the level sensing waveguide 20 into the reservoir, but at alevel that reduces or prevents reintroduction of the air bubbles in theDEF and reduces or prevents fluid with entrained air bubbles fromentering the level sensing waveguide 20.

If air vent 42 provided passage directly to the reservoir, such a ventwould provide a path for “micro-bubbles” to enter the sensing chamberduring fill and sloshing. According to another aspect of the presentinvention, the vent exit 42 may be protected by connecting it to thelevel sensing waveguide 20 (which vents to the top of the reservoirthrough vent 34) or a separate extension (“snorkel”) so that the ventoutlet is high enough in the reservoir to prevent aerated fluid fromentering the vent during a fill or slosh event. Venting into a snorkelor the waveguide has been found to be superior to known one-way valves(e.g. “duck bill” valve) at the vent. An issue with one way valves isthat they provide too much resistance due to cracking pressure for allthe air in the accumulator to exit. Venting to the level sensingwaveguide or to a snorkel provides a low resistance flow path whilestill preventing aerating fluid from entering the sensing chamber. Theair vent 42 in the concentration sensing chamber is a direct port intothe level sensing waveguide 20 to provide a shielded vent path.Additionally, FIG. 3 shows a fluid inlet 40 directly connecting theconcentration sensing housing 22 and the level sensing waveguide 20.This provides two benefits. First, the rate of liquid entering andexiting the level sensor waveguide is dampened by the porous media 32fluid path. This mitigates the effect of sudden changes in the liquidheight, while still allowing the sensor to track fill and drain ratesaccurately. The second benefit is that the lower port encourages theexchange of liquid in the concentration sensor during slosh and liquidlevel changes ensuring that the concentration sensor chamber is notisolated from the rest of the liquid in the tank.

In order to promote “wetting” of the reflector and therefore allow airbubbles to release for the reflector (or another mechanical item in thechamber), a hydrophilic coating is applied. This coating helps thereflector to shed bubbles (large and small).

It has been found that it may be advantageous to fluidly isolate theconcentration sensor chamber from the level sensing waveguide. Thisprovides greater control to address different needs for flow rates,fluid motion damping, and fluid dilution sensing. Also, if plain wateris added to the DEF reservoir, the fluid in the level sensing wave guidewill likely at least initially be at a higher urea concentration thanthe fluid in the rest of the DEF reservoir. Isolating the two sensingvolumes fluidly prevents the higher concentration fluid in the levelsensing waveguide from including an erroneous concentration measurementin the concentration sensing volume. Accordingly, another example of aDEF sensor assembly 50 is illustrated in FIGS. 4-6. Where componentsand/or features are the same between the examples, the same referencecharacters are used and the description of such components or featuresis not repeated.

A sensor housing 52 cooperates with a housing fluid port 54 and porousmedia 56 to define a concentration sensor volume 60 and separate levelsensor volume 70. A sensor reflector 64 is located in the concentrationsensor volume 60. Sensor reflector 64 is orientated vertically, and asensor 62 (shown in phantom lines) is located on the opposite end ofsensor volume 60. As before, “micro-bubble” attachment is mitigated bypreventing micro bubbles from entering the concentration sensor volume60.

The porous media 56 allows DEF fluid to fill the concentration sensorvolume 60 while substantially blocking micro-bubbles from entering thechamber. Porous media 56 may be mounted on fluid port 54. Fluid port 54may include a plurality of apertures 58 a, 58 b, 58 c to expose adesired area of porous media 56 to DEF fluid. The apertures 58 a, 58 b,58 c function as controlled-rate fluid entrances to the sensing chamber.Fluid port 54 may also have an O-ring style seal or other gasket toprevent ingress of bubbles around the frame. In another example, porousmedia 56 may be cut to approximately the same dimensions as fluid port54 and sandwiched between fluid port 54 and sensor housing 52.

To further assist in preventing air from entering the sensor volumes 60,70 via the porous media, a hydrophilic media or treatment applied to theporous media 56 may be utilized. In one example, the porous media maycomprise polyester felt media rated at 25 microns. The hydrophilicaspect of the media encourages the media to be fully “wetted” toencourage free flow of the liquid and to ensure that no air is trappedin the media. The sensor 62 is embedded in the sensor housing 52 andelectronically coupled to the controller 14.

While porous media 56 prevents bubbles from entering concentrationsensor volume 60, it also prevents air bubbles from exiting the volume,Accordingly, an air vent 66 provided at the top of the concentrationsensor volume 60 to allow air to escape the concentration sensor volume60 and return to the reservoir. The top of the concentration sensorvolume 60 may be angled or sloped to improve collection of trapped airand air bubbles.

In one advantageous aspect of the present invention, the top of theconcentration sensor volume 60 is sloped to guide bubbles away fromsensor reflector 64. An air accumulation volume 74 may also be providedfor the air to collect prior to exiting the chamber. The accumulationvolume 74 provides a space for air to collect away from the sensorcomponents. This way any remaining air that does not exit via the airvent 66 will not interfere with the sensor readings. The shape of theair accumulation volume 74 may vary, as long as it provides a guidancepath to the air vent.

As described above, the air vent 66 may be protected by connecting to anextension (“snorkel”) so that the vent outlet is high enough in thereservoir to prevent aerated fluid from entering the vent during a fillor slosh event.

A level sensor 72 (shown in phantom lines) is below level sensor volume70, which is coupled to level sensing waveguide 20. A waveguide vent 34is provided at the top of the level sensing waveguide 20 to allow air toescape from the level sensing waveguide 20 into the reservoir, but at alevel that reduces or prevents reintroduction of the air bubbles in theDEF and reduces or prevents fluid with entrained air bubbles fromentering the level sensor volume 70. The level sensor is electricallyconnected to the controller 14.

Operation of the concentration sensor 62 and sensor reflector 64 isrelatively unaffected by movement of fluid in and out of theconcentration sensor volume 60. Also, the concentration sensor needs toreact quickly to “dilution events,” that is, when plain water is addedto the DEF reservoir instead of the required aqueous urea mixture.Accordingly, a surface area of porous media 56 exposed by the aperturesin fluid port 54, fluidly coupling the reservoir to concentration sensorvolume 60 through porous media 56 may be relatively large. In theillustrated example, two apertures 58 a, 58 b, are provided. Thisencourages the exchange of liquid in the concentration sensor duringslosh and liquid level changes ensuring that the concentration sensorchamber is not isolated from the rest of the liquid in the tank.

Conversely, the fluid level sensor provides more accurate operation withhigher damping on flow rate of fluid in and out of the level sensorvolume 70. To reduce tank sloshing from rapidly affecting fluid levelswithin the wave guide, one relatively small aperture 58 c is provided influid port 54 to couple the reservoir to level sensor volume 70 throughporous media 56. In one example, the area of aperture 58 c is aboutone-tenth of the combined areas of apertures 58 a and 58 b. Because flowrates are proportional to the area of porous media 56 exposed by theapertures, level sensor volume 70 experiences much slower fluid fill anddrain rates as compared to concentration sensor volume 60. Theadditional damping of fluid flow rates into and out of the level sensorvolume 70 improves accuracy of fluid level measurements.

Another example of a DEF sensor assembly 100 including features of bothexamples above and further improving sensor response is illustrated inFIGS. 7 and 8. Where components and/or features are the same betweenexamples, the same reference characters are used and the description ofsuch components or features is not repeated.

A DEF concentration sensing housing 122 is provided. The concentrationsensing housing 122 comprises a DEF concentration sensor 24, a sensorreflector 126, and a housing fluid port 128.

Sensor reflector 126 is dimensioned to allow air to pass between it andat least one wall of concentration sensing housing 122, such as a rearwall.

A concentration sensing volume 138 is provided below the reflector andan air accumulation volume 136 is provided at the top of theconcentration sensing housing 122 above the sensor reflector 126. Airmay collect in the air accumulation volume 136 prior to exiting thechamber. The air accumulation volume 136 provides a space for air tocollect away from the sensor components. This way any remaining air thatdoes not exit via the vent 142 will not interfere with the sensorreadings. The shape of the air accumulation volume 136 may vary, as longas it provides a guidance path to the air vent 142.

According to one aspect of the present invention, “micro-bubble”attachment is mitigated by preventing micro bubbles from entering thesensing chambers, The housing fluid port 128 comprises an apertures 130a and 130 b, and porous media 132 extending across both apertures. Theporous media 132 allows DEF fluid to fill the concentration sensinghousing 122 through the aperture 130 a and the level sensing waveguide20 through aperture 130 b while substantially blocking micro-bubblesfrom entering either chamber. Apertures 130 a, 130 b may be subdivided,as illustrated with 130 b. Porous media 132 may be mounted on side ofthe housing fluid port 128 facing the sensing volumes. Porous media 132may be embedded in a gasket 134 to seal the porous media to the housingfluid port 128, sensing housing 122, and fluid port 140. To improvesealing a seal boss 146 may be included on the gasket 134, and acorresponding recess 148 included on fluid port 140. To further assistin preventing air from entering the chamber via the porous media 132, ahydrophilic media or treatment applied to the porous media 132 may beutilized.

A sensor 24 (illustrated in phantom lines) is located underneath theconcentration sensing housing 122 and electronically coupled to thecontroller 14 such that measurements are made vertically. To improveconcentration sensor response to changing DEF concentration in the DEFreservoir, the concentration sensor volume 138 is narrow relative to theopening of aperture 130 a and the area of porous media 132 such that anarea of a vertical cross section of the sensor volume 138 is smallerthan an area of aperture 130 a. Fluid port 140 couples aperture 130 b tolevel sensing waveguide 20. If the level sensing waveguide 20 is offsetfrom fluid port 140, an aperture 141 may be provided to complete thefluid path. Fluid port 140 and aperture 130 b are relatively small toreduce fluctuations in fluid levels in the level sensing waveguide 20.In the illustrated example, an area of a horizontal cross section of thelevel sensing waveguide is larger than an area of aperture 130 b and/oran area of a vertical cross section of fluid port 140.

An air vent 142 may be provided at the top of the concentration sensinghousing 122 and into the level sensing waveguide 20 to allow air toescape from the concentration sensing housing 122. The top of theconcentration sensing housing 122 may be angled or sloped to improveevacuation of trapped air and air bubbles. To prevent fluid in the levelsensing waveguide 20 from affecting DEF concentration measurements, airvent 142 is preferably small to minimize ingress of DEF from the volumesensing waveguide 120. In one example, air vent 142 is as small as maypractically be molded using conventional plastic molding techniques. Inthis way, the two sensing volumes are still effectively fluidlyisolated. The air accumulation volume 136 permits the air vent 142 to berelatively small by isolating unvented air on the opposite side of thetarget from the sensor.

Because the air vent 142 in the concentration sensing chamber 122 is adirect port into the level sensing waveguide 20 and the fluid in levelsensing waveguide 20 entered through the porous media and fluid port140, the air vent 142 provides a shielded path for the exhaust of airbubbles with low risk of reintroducing air bubbles from the main DEFreservoir.

Various features of the disclosed examples may be employed on thealternative examples. Also, while the present invention is beingdescribed with respect to sensors that utilize ultrasonic sensing, theconcept or air removal applies to multiple sensor types. With sensingtechnologies that utilize a reflector as depicted above, the size of thereflector must allow for air to easily pass by from the lower (sensing)volume to the upper accumulation volume.

What is claimed is:
 1. A housing assembly for a fluid sensor assembly,comprising: a housing having a first sensing volume and a second sensingvolume; a fluid port attached to the housing and having at least a firstfluid aperture and second fluid aperture; at least one porous membraneassembled between the fluid port and the housing, the at least oneporous membrane covering the first and second fluid apertures; the firstsensing volume comprising a vertically-oriented waveguide, the firstsensor volume being fluidly coupled to an exterior of the housingthrough the first fluid aperture, the first aperture port having an areasmaller than an area of a cross section of the waveguide; and the secondsensing volume being coupled to the exterior of the housing through thesecond fluid aperture, the second fluid aperture having an area largerthan an area of a cross section of the second sensing volume; whereinthe fluid port and porous membrane internally isolate the first sensingvolume from the second sensing volume.
 2. The housing assembly of claim1, wherein the fluid port is planar.
 3. The housing assembly of claim 1,wherein the at least one porous membrane comprises a single sheet ofporous membrane covering the first and second fluid apertures.
 4. Thehousing assembly of claim 1, further comprising a gasket disposed on theporous membrane.
 5. The housing assembly of claim 4, wherein the gasketinternally isolates the first sensing volume from the second sensingvolume.
 6. The housing assembly of claim 1, wherein the fluid port isplanar and the at least one porous membrane comprises a single sheet ofporous membrane covering the first and second fluid apertures
 7. Thehousing assembly of claim 6, further comprising a gasket disposed on theporous membrane, wherein the gasket internally isolates the firstsensing volume from the second sensing volume.
 8. The housing assemblyof claim 1, further comprising a first ultrasonic sensor under thewaveguide, a second ultrasonic sensor at a bottom of the second sensingvolume, and a target located a fixed distance above the secondultrasonic sensor.
 9. The housing assembly of claim 1 wherein the firstsensing volume further comprises a first vent to the exterior of thehousing assembly at a top of the waveguide, and wherein the secondsensing volume further comprises a second vent into the first sensingvolume located at a top of the second sensing volume.
 10. A housingassembly for a fluid sensor assembly, comprising: a housing having afirst sensing volume and a second sensing volume; the first sensingvolume comprising a vertically oriented waveguide having a first sensorat a bottom of the waveguide and a first air vent to an exterior of thehousing assembly at a top of the waveguide, the first sensor volumebeing fluidly coupled to an exterior of the housing through a firstfluid aperture covered by a first area of porous membrane; and thesecond sensing volume having a target at a fixed distance from a secondsensor and a second air vent at a top of the second sensing volume, thesecond air vent venting into the first sensor volume, the second sensingvolume being coupled to the exterior of the housing through a secondfluid aperture covered by a second area of porous membrane, wherein thefirst and second apertures do not provide internal fluid communicationbetween the first sensing volume and the second sensing volume.
 11. Thehousing assembly of claim 10, wherein the target s located verticallyabove the second sensor.
 12. The housing assembly of claim 10, whereinthe second sensing volume further comprises an air accumulation volumeabove the target.
 13. The housing of claim 12, wherein the second airvent is horizontal and provides a passage into the waveguide from theair accumulation volume.
 14. The housing assembly of claim 10, whereinthe first area of porous membrane and the second area of porous membraneare provided on a single sheet of porous membrane covering the first andsecond fluid apertures.
 15. The housing assembly of claim 10, whereinthe first and second fluid apertures are provided on a planar fluidport.
 16. The housing assembly of claim 10, wherein the first and secondfluid apertures are provided on a planar fluid port, the first area ofporous membrane and the second area of porous membrane are provided on asingle sheet of porous membrane covering the first and second fluidapertures, and the sheet of porous membrane is attached to the housingassembly by the fluid port.